In recent years, hybrid cars, electric vehicles are being spread. A driving motor is installed in such a vehicle, and the secondary battery is also installed which supplies power to the driving motor and stores power regenerated from the driving motor. Generally, lithium ion batteries, nickel hydride batteries are used for the secondary battery for the vehicle. As a power supply system for the vehicle, a power supply system is proposed where different shapes of batteries are connected in parallel. For example, the power supply system is proposed where the prismatic battery and the cylindrical battery are connected in parallel (for example, refer to Patent Literature 1). When the prismatic battery and the cylindrical battery are compared, the prismatic battery has high output, and the cylindrical battery has high energy density. Accordingly, both batteries are connected in parallel, and then the assembled battery has high output and high energy density.
While, in the secondary battery, the high rate discharge accelerates degradation. When the secondary battery is discharged at high rate, its voltage is apt to irregularly change. It is possible to improve tolerance against the high rate discharge by appropriately selecting material composition of the battery, but it causes cost increase, decrease of batter property.
When a parallel system, where the secondary batteries having different shapes are connected in parallel, is designed, generally the capacity and the parallel-connected number of the secondary batteries in each of the shapes are determined based on the internal resistance ratio between the secondary batteries of the different shapes, such that the discharge rate in each of the secondary batteries becomes 4 to 8C or less. Here, the actual limit value of the discharge rate is generally equal to or less than 4C in the battery for a consumer usage, depending on material composition of the battery.